Moa

Started September 26, 2021

Moa is an emulator/simulator for computers using various 68000 and Z80 CPUs and peripherals. The original idea was to emulate the computer I had built as part of the Computie project.

Currently it can simulate the Sega Genesis, Computie (68000), and the TRS-80 Model I (Z80). Support for the Macintosh 512k is partially implemented but the ROM still wont boot.

For more details on how it works, check out this post about how I started the project: Making a 68000 Emulator in Rust

For more about the Sega Genesis support, check out this series I wrote about implementing it: Emulating the Sega Genesis

Sega Genesis/MegaDrive

From the project root, run the following:

cargo run -p moa-minifb --release --bin moa-genesis -- <ROM FILE>

The Genesis emulator is slowly coming along. It can play a decent number of game, but some games wont display anything, and a few games run but don't respond to the controller input. Games that require extra memory or nvram that would normally be inside the cartridge usually crash.

It only supports NTSC mode at the moment, and only VDP mode 5 (not the backwards compatible mode 4). I've rewritten the frame drawing code to operate pixel by pixel, so it will now draw all the layers, including the window, sort out the priority of the pixels, and almost accurately implement the shadow and highlight colour modes. Audio is not implemented yet.

There are still some problems like the colour of Tails in the Sonic 2 title screen being off. I'm not sure why that happens, but it could be trying to update the colours during the drawing of the frame, and since the code is drawing the entire frame at once when the vertical blanking period is reached, the on-the-fly changes don't have an affect. I've since fixed the colour issue with Tails by fixing some 68000 instruction behaviour with the help of Tom Harte's test suite.

The game play is mostly working but the time in the upper left corner doesn't seem to progress

Earthworm Jim was working before I fixed the controller behaviour in Sonic 2, which seems to have broken it in Earthworm Jim (and Mortal Kombat 1).

The following gif was only recorded at 15 frames a second instead of the full 60, so it appears jerky and some animations seem to freeze in the gif, even though that doesn't happen during normal play

Computie

For Computie, it can do everything the 68k-SMT board can do, including run the monitor program and load the Computie OS kernel and boot it from the CompactFlash card. To run it:

cargo run -p moa-console --bin moa-computie

It will open two PTYs: one for the serial terminal, and one for the SLIP connection. Once open, it will try to launch both pyserial-miniterm as a separate process and connect to the Computie PTY. It will also launch slattach with the associated setup commands to create the SLIP device on the host, and set up host routing. The exact commands in src/machines/computie.rs might need to be adjusted to work on different hosts.

TRS-80

For the TRS-80, it can run Level I or Level II Basic, but it doesn't yet support a cassette tape drive or floppy drive. I haven't tested it that thoroughly either, so any help with it would be welcome. I mostly made it to test the Z80 cpu implementation in a simpler computer before I used in the Genesis emulator. The frontend uses the minifb rust crate to open a window and render the characters to screen, as well as accept input from the keyboard.

cargo run -p moa-minifb --release --bin moa-trs-80

By default it will start Level I Basic. To use the other rom, add the option --rom binaries/trs80/level2.rom

The characters are being drawn pixel by pixel (6x8) using characters I drew using this handy website. They aren't a perfect match of the characters used by the TRS-80

General Options

By default, the minifb frontend will scale the window by 2. This can be changed with the --scale [1,2,4] option.

The -t or --threaded options will run the simulated hardware in a separate thread from the frontend, which will run as fast as possible, faster than real-time. By default, the simulated hardware is run inline with the frontend's update cycle, which is limited to 60Hz. The simulation will be run for 16.6ms of simulated time for each frame the frontend draws. But the simulated time is not accurate and Sega Genesis games will run slower than they should.

The -d or --debugger option will make the emulator start the debugger before running. There is a simple built-in debugger for stepping through the rom instructions being emulated. The state of the CPU registers will be displayed after each instruction, breakpoints can be set, memory contents can be examined, and memory locations can be modified. This has helped a lot with tracking down errors in the emulator itself.

The -x or --speed option, when given a decimal number, will multiply that number by the milliseconds per frame, increasing or decreasing the gameplay clock relative to the frontend's update loop. Setting it to 0.5 slows the game down to half speed and setting it to 2 doubles the speed.

The -a or --disable-audio option will prevent the audio device from being created, so no audio will be played (although it will still be simulated by any devices that simulate it).